Miliage gone to hell, need input

As of late, the miliage in my E46 325i has gone down the shitter. I used to average over 300 miles per tank, with pretty hard driving (heavy accleration, etc.) That was with 91 octane. With 93, I could get just over 350 before the light came on. Past few weeks, the 'rents have filled up with 87 sloptane, but the past two tanks I've put in have been 93. Just got the oil changed, Mobile 1 synthetic. I have actually been driving very soft lately.

However, starging about three weeks ago, I started getting crap miliage. Barely getting 250 miles out of a tank, and that was pushing it to the end of the tank. I thought with puting the 93 back in, it would bump it up, but thats not the case. I checked all four tires, pressure is right where it should be.

At this point, I think it is one of three situations:

1)I was 500 miles past due when I got the oil change, so my thought was that the computer was messing with the Air fuel ratio with the worry that the oil was going bad. When I got the oil changed, they didn't have the computer to reset the oil change meter. So while it says that I am about 700 past do, in reality, it already had the oil change. My thought is that computer is still messing with the engine to keep it from getting hurt, even though there is fresh oil in it.

2)I need to get the fuel system flushed and cleaned. I'm at 46k (its a 2004 325i, at 46k, ugh), and unless it was done by BMW at Service 2 at 35k miles, it hasn't been done yet.

3)The computer is getting used to the motor getting the good stuff again, and after maybe another tank of 93, it'll be back to normal.

Also, I've noticed it being a little more slugish than usual under hard acceleration, and I've been attributing this to situation 2 or 3. Any ideas?

This one is kind of hard to tell. Have you tried disconnecting the negative on your battery to reset the computer? Im not sure if it works with the E46's, but it usually works with my buddies E36 if I recall correctly. It may be worth a shot, and try it out.

uh, I wouldn't count on any of your ideas being right. The computer shouldn't make fuel/timing changes based on oil life. But I have a couple of my own thoughts.
1. maybe its time for a tune up?? new plugs, fuel injector cleaner, water drier, ect.
2. maybe you have an injector leaking a bit. Check your plugs to see if its running really rich.
3. Have you checked the codes? its pos. that your 02 sensor or engine coolant sensor have gone south. either will cause poor fuel economy.
does it idle properly and run fine other than being a bit sluggish?

does it idle properly and run fine other than being a bit sluggish?

Yup, no problems with idle and run at all. Maybe its just getting used to the hot weather again, I've been in 80 or below Missouri since last July, and now I'm back to "Well, if it gets below 99, we may have a cold front comming through" Texas.

Regarding the tune up, I had the Service 2 done not too long ago. However, I have no idea what they do in the services at BMW, because the dealership where I've taken it has been a total dick to me. Anytime I have to bring it in to the place where I bought it, the service dept. is awesome about letting me stay in the area and watch, and stuff like that. Where I've had to take it for Service 1 and 2, in MO, they are like, no, you cant be in here, go wait outside.